1. FIELD OF THE INVENTION
The invention relates to ferrule-type seals for rigid metal tubing which is harder than the ferrule and, more particularly, to a roll-formed seat and retainer on such a tube which can be used with a ferrule to form a fluid-tight seal, and a tool and method for forming the seat.
2. DESCRIPTION OF THE RELATED ART
The present inventions relate to ferrule-type seals on rigid metal tubes where the tubes are formed of a material that is harder than the ferrule. Commercially-available ferrule fittings are specified for use with rigid metal tubes with a hardness that is lower than the cold-worked, high-strength hardness of most stainless-steel alloys. Thus, ferrule fittings are not used on these types of tubes where high internal tube pressure or high tension loads would push or pull the tube out of the fitting. Thus, the use of ferrule fittings in these types of applications is not recommended.
There are many potential uses of ferrule tube fittings where the metal tube is harder than the ferrule. However, ferrule fittings for such uses are limited because the ferrule cannot deform the tube or bite into the outer surface of the tube to form a seal. Instead, the ferrule deforms against the outer surface of the tube. This type of seal does not have the ability to withstand high pressure or tension forces in the tubes because the ferrule and its associated fittings will blow or pull off the tube when the applied pressure or tension exceeds a certain level.
One application for ferrule fittings where tubes are harder than commercially-available ferrules is in wirelines that are used in conjunction with logging or measurement instruments that are typically lowered into an underground well, such as an oil or gas well, to be positioned adjacent to a subterranean formation to measure pressure, temperature and other downhole characteristics. The diameter of a typical wireline tubing of the type described ranges from about 1/8" to 1/2. While various types of materials may be used to form the rigid tube, the preferred material is corrosion-resistant with relatively high strength and hardness at elevated temperatures which is accomplished by cold working the material during manufacture. Various types of stainless steel known to those skilled in the industry are typically used.
A seal must be formed in the connection between the wireline tubing and the downhole measuring tool to prevent leakage of well fluids into the tube which causes electrical short circuits. While such a fitting typically uses a ferrule as part of the fitting, even though the ferrule can deform into intimate contact between the tubing and connector walls, the ferrule is unable to bite into the outer surface of the metal tube because the tube is "work hardened" when it is formed into a tube and resists deformation.
Thus, a need exists for a fluid-tight, ferrule-type seal on tubes which is harder than the ferrule, which retains the ferrule on the tube and resists failure at high pressures and tension loads. The seal should be one that is formed quickly and easily in the field without the use of complicated or cumbersome tools or machines.
Another problem that can arise in the use of wirelines occurs when the tool is raised out of a well bore. Because of irregularities in the well bore or other problems, it is not uncommon for a downhole tool to get stuck. When that occurs, continued spooling of the wireline at the surface causes increased tensile force along the length of the wireline which, if not stopped, causes the wireline to break. If a break occurs, the tool as well as the length of the wireline tubing beneath the breaking point remain in the bottom of the well.
Fishing operations for retrieving a tool are not easy to perform. However, they are made more difficult and time consuming when a length of wireline is positioned above a tool because the wireline hampers efforts to grasp the tool. Additional time necessary to retrieve a tool translates into downtime for the well logging operations and dramatically increases the cost of the well.